共查询到20条相似文献,搜索用时 15 毫秒
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A rational approach to the design of gas-fired finned tube heat exchangers is presented. As a result of an extensive literature search and a study of flue gas properties, novel equations have been developed to facilitate design procedures. Comparisons are made between the theoretical performance of a finned tube boiler and those determined experimentally. 相似文献
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Heat transfer improvement in a water wall tube with fins was investigated in a circulating fluidized bed (CFB) boiler. Experiments were first conducted in a 6 MWth CFB boiler then a model was developed to analyse and interpolate the results. Temperatures at some discrete points within the wall cross‐section of the tube were measured by burying 0.8 mm thermocouples within a tube. Experimental data showed an increase in heat absorption up to 45 per cent. A good agreement between measured and predicted values was noted. The distribution of temperature in the metal wall and of heat flux around the outer wall of a tube with longitudinal and lateral fins was analysed by numerical solution of a two‐dimensional heat conduction equation. Effects of bed‐to‐wall heat transfer coefficient, water‐to‐tube inside heat transfer coefficient, bed temperature, water temperature and thermal conductivity of the tube material on the heat flux around the water tube are discussed. The present work also examines the influence of the length of the longitudinal fin and the water tube thickness. Heat flux was highest at the tip of the longitudinal fin. It dropped, but increased again near the root of the lateral fin. Copyright © 2000 John Wiley & Sons, Ltd. 相似文献
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《International Communications in Heat and Mass Transfer》2005,32(3-4):557-564
This work presents a numerical investigation of a vertical internally finned tube subjected to forced convection heat transfer. The governing equations were solved numerically using the control volume technique. Nusselt number, Nu, and friction factor multiplied by Reynolds number, fRe, are influenced greatly by the height and number of the radial fins. The velocity and temperature distributions inside the tube depend on the number and height of the radial fins. This paper suggests that for best heat transfer to be achieved there is an optimum combination of fin numbers and height. 相似文献
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基于Nusselt凝结传热理论,沿肋片管圆周方向划分有限个微元角,建立了每个微元角内肋侧壁、肋间基管及肋顶三个区域的凝结传热模型,通过求解非淹没区和淹没区总传热量,推导管外平均传热系数计算式。计算不同肋片高度、肋密度时,R134a饱和蒸汽的管外平均凝结传热系数。结果表明:随肋密度的增加,平均传热系数先增大后减小,肋密度为25fpi时传热最佳;高肋片管的平均凝结传热系数大于低肋片管的,肋片高度达到一定值时,平均传热系数几乎不随肋高增加而增加。当R134a饱和蒸汽为20℃时,两种不同翅片密度的管外平均凝结传热系数随温差的增大而减小,并通过所建模型得到的计算值与Beatty-Kate模型进行了比较,平均误差分别为约16.1%和8.3%,故所建模型基本反映肋片管外蒸汽凝结传热机理。 相似文献
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Four different semi-empirical models of heat transfer and pressure drop for helically segmented finned tubes in staggered layout were analyzed. The performance of a Helically Segmented Finned Tubes Heat Exchanger on an industrial scale was obtained and the predictions were compared with experimental data. The method used for thermal analysis is the Logarithmic Mean Temperature Difference (LMTD). Comparisons between predictions and experimental data show a precision greater than 95% in heat transfer for a combination between the Kawaguchi and Gnielinski models at a flue gas Reynolds number, based on the outside bare tube, of about 10,000. In the case of pressure drop, there is a precision of approximately 90% for the Weierman model at a Reynolds number, based on the outside bare tube, of about 10,000. And so, the results show that the best flow regime in which heat transfer and pressure drop are optimum, is for a Reynolds number (based on the outside bare tube) of about 10,000. 相似文献
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《International Journal of Heat and Mass Transfer》1987,30(2):363-372
A finite volume numerical scheme is utilized to predict fluid flow and heat transfer characteristics in inline tube banks. The effect of equipping the tubes with longitudinal fins on the pressure drop and heat transfer is studied. The governing equations for fluid flow and heat transfer are numerically solved, with the assumption of periodic, fully developed flow. The numerical methodology utilizes the stepped boundary technique to approximate the tube surface. The tubes are maintained at a constant temperature, and the calculations are carried out for laminar flow and for a large range of Reynolds and Prandtl numbers. The results for the unfinned tube case are compared with previously published experimental data. The numerical results agree well with the experimental measurements. Representative results for the case of the finned tubes indicate, surprisingly, a decrease in the heat transfer rate, and small changes in the pressure drop, as a result of finning. The decrease in the heat transfer rate probably occurs because the fins are placed in the stagnation regions at the front and rear of the tubes, and thus do not increase the heat transfer. 相似文献
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采用数值模拟方法,对径向错列翅片管内含不凝结气体水蒸气的凝结对流换热及阻力特性进行了综合分析。将编写的自定义函数(UDF)导入ANSYS FLUENT软件,对新型强化管传热性能和阻力性能进行了数值模拟,并根据管长方向壁面上蒸汽质量分数的变化情况,讨论分析了凝结过程中翅片管传热性能的变化规律。分析结果表明:与光管相比,内翅片管的强化传热效果随翅数增多、翅片换热接触面积增大而更加显著;另一方面,翅片管的流动阻力相应增大,对管路换热产生不良影响。在所研究翅型范围内16翅y=2x~2型翅片管综合强化换热效果更优;此外随着换热过程的持续,蒸汽凝结逐渐放缓;入口速度增大导致水蒸气凝结不充分,对换热效果的提升有一定制约。 相似文献
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《International Journal of Heat and Mass Transfer》2007,50(7-8):1231-1237
An experimental study of heat transfer performance in 3D internally finned steel-water heat pipe was carried out in this project. All the main parameters that can significantly influence the heat transfer performance of heat pipe, such as working temperature, heat flux, inclination angle, working fluid fill ratio (defined by the evaporation volume), have been examined. Within the experimental conditions (working temperature 40 °C–95 °C, heat flux 5.0 kw/m2–40 kw/m2, inclination angle 2–90°), the evaporation and condensation heat transfer coefficients in 3D internally finned heat pipe are found to be increased by 50–100% and 100–200%, respectively, as compared to the smooth gravity-assisted heat pipe under the same conditions. Therefore, it is concluded that the special structures of 3D-fins on the inner wall can significantly reduce the internal thermal resistance of heat pipe and then greatly enhance its heat transfer performance. 相似文献
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《Energy》2001,26(11):963-972
The effect of the number of tube rows on heat, mass and momentum transfer is experimentally investigated for flat-plate, finned-tube heat exchangers which consist of aluminum fins and copper tubes. Four flat-plate finned-tube heat exchangers are identical except for changes in the number of tube rows (1 to 4). Heat-transfer coefficients for wet and dry surface conditions are obtained for both heating and cooling of moist air flowing over finned tubes. The air velocity was varied from 0.9 to 4 m/s. Heat transfer, Colburn and friction factors are determinated for different tube rows numbers while the Reynolds number were being warried. It is found that the values of Colburn and friction factors for wet surfaces are higher than for dry surfaces and for both conditions the Colburn and friction factors decrease with an increase in the tube row numbers. 相似文献
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Il Seouk Park Man Young Kim 《International Journal of Heat and Mass Transfer》2009,52(11-12):2599-2606
A numerical study for the flow, heat and mass transfer characteristics near the inflow region of the vertical evaporating tube with the films flowing down on both the inside and outside tube walls has been carried out. Condensation occurs along the outside wall and evaporation at the free surface of the inside film. The transport equations for momentum and energy are parabolized by the boundary-layer approximation and solved by using the marching technique. In this kind of numerical approach, the accurately predicting the early stage is really important because a small error at the previous step can produce the amplified big error at the next step. To accurately predict the flow at the inflow region of the vertical evaporating tube, the calculation domain of two film flow regions and tube wall is solved simultaneously. The interesting heat transfer characteristics revealed through this three-zone simulation, such as the evaporation delay and the temperature inflection at the very near inflow region are found and discussed along the discrepancy between the inner film inlet temperature and the saturation temperature. The case that the inner film comes in with the saturation temperature shows a good performance. The velocity and temperature fields as well as the amounts of the condensed and evaporated mass in both inner and outer films are predicted for the various conditions. 相似文献
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The present work submits an experimental work on the heat transfer and friction loss characteristic, employing a tube finned heating surface kept at a constant temperature in a rectangular channel. The tube fins attached on the surface (o.d.=29 mm) were arranged as either in‐line or staggered. The parameters for the study were Reynolds number (3700–30 000), depending on hydraulic diameter, the distance between the tube fins in the flow direction (Sy/D=1.72–3.45) and the fin arrangement. The change in the Nusselt number with these parameters was determined. For both tube fin arrangements, it was observed that increasing Reynolds number increased Nusselt number, and maximum heat transfer occurred at Sy/D=2.59. Thermal performances for both arrangements were also determined and compared with respect to heat transfer from the same surface without fins. With staggered array, a heat transfer enhancement up to 25 per cent for Sy/D=3.45 in staggered array was achieved in constant pumping power. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献
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对采用临界喷嘴式分流器的冷库用翅片蒸发器性能进行实验研究,在0、-4、-8、-12、-16、-18及-20℃七种不同环境工况下进行传热性能测试,并将测试结果与采用气液分离式分流器、CAL分流器和文丘里式分流器进行对比分析。研究结果表明:库温为0℃时,对应的翅片式蒸发器的制冷量和传热系数分别为8.5 kW和37.9 W/(m~2·℃),相比气液分离式分流器、德国CAL分流器、文丘里分流器其对应冷风机的制冷量分别提高了8.1%、17.4%和21.7%,传热系数提高了4.5%、13.9%和18.6%。库温为-18℃时,对应的翅片式蒸发器的制冷量和传热系数分别为5.5 kW和30.9 W/(m~2·℃),其对应的蒸发器的制冷量分别提高了7.2%、14.3%和18.7%。库温为-18℃时,对应的传热系数分别提高了0.9%、10.1%和13.2%。实验结论为有效提高翅片蒸发器性能提供了一种解决方案。 相似文献
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Stephen Sharratt Cheng Peng Y. Sungtaek Ju 《International Journal of Heat and Mass Transfer》2012,55(21-22):6163-6169
We report a modeling and experimental study of micro-post wicks with improved phase change heat transfer performance for evaporators in micro-heat pipes and vapor chambers. A surface energy minimization algorithm is used to predict the shapes of liquid menisci around micro-posts whose geometries have been tailored to increase the fraction of thin-film regions with small thermal resistance. The effects of the apparent contact angle, solid thermal conductivity, and accommodation coefficient are studied. A circular post of uniform cross-section is used as a baseline to evaluate the performance of two alternative post geometries with the same virtual diameter and array spacing. Enhancement in the heat transfer coefficient due to increased fractions of thin-film regions is largest when the relative effects of the liquid conduction or evaporation resistance are large, that is, when the apparent contact angle is large or when the solid thermal conductivity is high. Deep reactive ion etching or electroplating was used to fabricate wick structures out of silicon or copper with aspect ratios as high as 14. The heat transfer performance of the wicks are experimentally tested in a controlled ambient and the effective heat transfer coefficient is determined using a 3D heat transfer model to account for heat spreading. At low and moderate heat fluxes (<20 W/cm2), the experimentally measured performance is consistent with our modeling results. 相似文献
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《International Communications in Heat and Mass Transfer》2005,32(3-4):371-385
In the present study, the average tube-side and air-side heat transfer coefficients in a spirally coiled finned tube heat exchanger under dry- and wet-surface conditions are experimentally investigated. The test section is a spiral-coil heat exchanger, which consists of six layers of concentric spirally coiled tube. Each tube is fabricated by bending a 9.6-mm outside diameter straight copper tube into a spiral coil of four turns. Aluminium fins with thickness 0.6 mm and outside diameter 28.4 mm are placed helically around the tube. The chilled water and the hot air are used as working fluids. The test runs are done at the air and water mass flow rates ranging between 0.02 and 0.2 kg/s and between 0.04 and 0.25 kg/s, respectively. The inlet-air and -water temperatures are between 35 and 60 °C and between 10 and 35 °C, respectively. The effects of the inlet conditions of both working fluids flowing through the heat exchanger on the heat transfer coefficients are discussed. New correlations based on the data gathered during this work for predicting the tube-side and air-side heat transfer coefficients for the spirally coiled finned tube heat exchanger are proposed. 相似文献